In the field of retrofit lamps for replacing incandescent lamps by light units having a lower power consumption and a longer lifetime, solutions are demanded to provide a pleasant atmosphere and a behavior comparable to the incandescent lamps to be replaced.
The color of the light emitted by incandescent lamps is dependent on the electrical power provided to the lamp so that the color of the emitted light becomes warmer when the incandescent lamps are dimmed. This color shift during dimming is appreciated by the user of incandescent lamps since the warmer white provides a more pleasant atmosphere.
Since retrofit lamps which are based on LEDs do not have a color shift of the emitted light during dimming, additional effort is necessary to emulate the dimming behavior of incandescent lamps.
A simple and commonly known solution is to add amber-colored LEDs to the white LEDs which are activated when the retrofit lamp is dimmed to a predefined dimming level. By means of the amber LEDs, the emitted light is shifted to a warmer white color. However, in addition to the additional amber LEDs, a circuitry is necessary to balance the current between the white and the amber LEDs to obtain a proper color point, so that the overall technical effort of these retrofit lamps is increased.
A possibility to distribute the current between the white LEDs and the amber LEDs is to provide a string of white LEDs and a parallel string of amber LEDs and to control the current of the amber LEDs by means of a transistor as disclosed in WO 2010/103480 A2. The disadvantage of the solution is that the control of the current ratio is difficult and expensive since low voltages need to be measured and amplified with low offset voltage operational amplifiers so that the overall technical effort is large.
US2013/0063035 also discloses a light unit provided with in parallel a string of white LEDs and a string of amber LEDs. The current in the string with amber LEDs is controlled by a current regulator. In a simple configuration, the current regulator may keep the current in the string with amber LEDs constant. However this does not provide a dimming behavior comparable to an incandescent lamp. Alternatively the current regulator may be controlled by a microprocessor in order to simulate the dimming curve of an incandescent lamp during dimming. The use of a microprocessor is complicated and expensive.